1. Field of the Invention
The present invention relates to a structure and method for connecting a flat electrical cable to bus bars. More particularly, the invention concerns a connection between relaying bus bars, and a flat cable used for steering equipment in automobiles. In automobiles, a flat cable is contained in a cable reel. The flat cable is then connected to lead cables joined to external circuits, through the relaying bus bars.
2. Description of Background Information
FIGS. 1 and 2 show a ring-shaped cable enclosure including a fixed element 1 joined to a steering shaft. The cable enclosure further includes a mobile element 2 connected to a steering wheel in a freely rotatable way around the fixed element 1. A flat cable 3 is then stored in the cable enclosure in a helically wound condition. The flat cable 3 has a given length and each of its two ends is connected to an edge of a relay bus bar 5 attached to inner cases. The latter are respectively joined to the fixed element 1 and the mobile element 2. The other edge of the relay bus bar 5 is connected to lead cables 6 leading to external circuits. The flat cable is wound or unwound in unison with the rotation of the mobile element 2.
As shown in FIG. 2, when connecting the flat cable 3 to the relay bus bar 5, a plurality of relay bus bars 5 having a narrow width are fitted into grooves aligned in the inner case 4, and insert-molded with the latter. An end portion of the flat cable 3 is stripped of its resin coating, so that aligned conductor strips 3a are exposed. These conductor strips 3a are superposed on the plurality of relay buys bars 5, and are bonded by ultrasonic welding. A lid 7 us then put on the inner case 4, such that the joint section is protected.
As the number of circuits increases, the widths of the conductor strips 3a of flat cable 3 are reduced, as are the setting-up pitches of the conductor strips 3a. Consequently, the connection between the conductor strips 3a and the relay bus bars 5 are mechanically weakened, to the extent that the joint section can no longer be protected solely by the inner case 5 and the lid 7. Further, the ribs formed by the neighboring grooves of inner case 4, which isolate the conductor strip layers from each other, are also rendered smaller and narrower, so that the circuits can no longer be insulated sufficiently from one another.
As described above, when the relay bus bars 5 are insert-molded into the inner case 4, the relay bus bars 5 are made smaller. Their mechanical strength is thus reduced. Accordingly, the relay bus bars 5 are easily broken during insert-molding. Moreover, their fixing positions in the inner case 4 become more prone to variations.
Further, manufacture of dies for insert-molding relay bus bars 5 with an inner case 4, and for a protective lid 7, incurs high costs.
In view of the above, a primary object of the present invention is to provide a joint section between a flat cable and bus bars which is securely protected and insulated, and to reduce production costs.
To this end, there is provided a structure for connecting a flat cable to bus bars, the flat cable exposing conductor strips. The structure includes bus bars and conductor strips adhered onto the bus bars, thereby forming a joint section including strip layers and strip gaps. The structure further includes a first and second insulator resin sheet placed respectively on a first and a second face of the joint section. In this structure, at least the first insulator resin sheet is configured such that it penetrates into the strip gaps and adheres onto the second insulator resin sheet, so as to form insulating grooves.
Preferably, at least the first insulator resin sheet is a thermoplastic insulator resin sheet. Further, the conductor strips may be adhered onto the bus bars by welding. Suitably, the strip layers includes at least one of the configurations chosen from the groups consisting of punched holes, embossed figures and rivet cramping.
The structure may further include a holder and a lid made of an insulator resin and enclosing the joint section including the first and the second insulator resin sheet.
There is further provided a method of connecting a flat cable to bus bars including preparing bus bars in the form of strips, stripping off a coating from an end portion of a flat cable, whereby conductor strips are exposed, and adhering the conductor strips onto the bus bars by welding, thereby forming a joint section having a first face and a second face, and including strip layers and strip gaps. The method also includes covering the first face with a thermoplastic insulator resin sheet, while covering the second face with an insulator resin sheet, respectively, providing a device including a pressing plane with alternating concave and convex configurations, and heating the device and pressing the pressing plane onto the thermoplastic insulator resin sheet, whereby the thermoplastic insulator resin sheet adheres to the strip layers by virtue of the concave configurations, while it adheres to the insulator resin sheet in the strip gaps by virtue of the convex configurations, thereby forming insulating grooves.
Alternatively, there is provided a method of connecting a flat cable to bus bars including preparing an insulator resin sheet, arranging bus bars in the form of strips on the insulator resin sheet, stripping off a coating from an end portion of a flat cable, whereby conductor strips are exposed, and adhering the conductive strips onto the bus bars by welding, thereby forming a joint section having a first face and a second face, and including strip layers and strip gaps. The method also includes covering the first face with a thermoplastic insulator resin sheet, while covering the second face with an insulator resin sheet, respectively, providing a device including a pressing plane with alternating concave and convex configurations, and heating the device and pressing the pressing plane onto the thermoplastic insulator resin sheet, whereby the thermoplastic insulator resin sheet adheres to the strip layers by virtue of the concave configurations, while it adheres to the insulator resin sheet in the strip gaps by virtue of the convex configuration, thereby forming insulating grooves.
Alternatively yet, there is provided a method of connecting a flat cable a bus bars including preparing bus bars in the form of strips, stripping off a coating from an end portion of a flat cable, whereby conductor strips are exposed, and adhering the conductor strips onto the bus bars by welding, thereby forming a joint section having a first face and a second face, and including strip layers and strip gaps. The method also includes covering the first face and the second face with a thermoplastic insulator resin sheet, respectively, providing a first device and a second device, respectively, including a pressing plane with alternating concave and convex configurations, and heating the first and second devices and pressing the pressing plane onto the first and second faces, respectively, whereby the thermoplastic insulator resin sheets adhere to the strip layers by virtue of the concave configurations, while they adhere to each other in the strip gaps by virtue of the convex configurations, thereby forming insulating grooves.
The above methods may further include enclosing the joint section covered with the thermoplastic insulator resin sheet and/or the insulator resin sheet, with a holder and a lid.
Suitably, the insulator resin sheet is made of polyethylene terephthalate, and has a thickness of about 70 xcexcm.
As described above, a joint section (including welded strip layers), that includes bus bars and conductor strips aligned in parallel, is arranged on an insulator resin sheet (base plate). Another insulator resin sheet is then placed on the joint section and spread thereon. As the welded bus bars and conductor strips are covered with an insulator resin sheet from both sides thereof, the welded strip layers are mechanically reinforced by the insulator resin sheet, and protection of these strips is greatly improved. Further, both lateral faces of each of the welded strip layers are likewise covered with the insulator resin sheet. Furthermore, the insulator resin sheet (conductor strip side) is also adhered to the portions of the base plate placed between the welded strip layers, thereby forming an insulating section. The insulating section improves the insolation between the circuits. Accordingly, it is no longer needed to insert-mold the bus bars in an inner case, or to manufacture a molded cover, as done in the past. Production costs can thus be greatly reduced.
According to the above invention, one face of the insulator resin sheet which is put into contact with the joint section is preferably coated with an adhesive.
In a first connecting method, the bus bars are not loaded on the base plate at the outset. Instead, the conductor strips of a flat cable are first superposed on the bus bars. Thereafter, they are bonded by ultrasonic welding or a similar means, and then placed on the base plate (one of the insulator resin sheets). In this method, both ends of the aligned bus bars may be joined beforehand by a respective carrier band. In this condition, the conductor strips are superposed on the bus bars and welded. After welding, the carriers are cut off, to free the circuits.
In a second connecting method, the bus bars are first placed on the base plate. The conductor strips of a flat cable are then superposed on the bus bars. Subsequently, the bus bars and the conductor strips are bonded by ultrasonic welding or a similar means. In this method, the base plate may be provided with small holes arranged at a given interval. The bus bars and the conductor elements are then arranged on the small holes, and welding is performed.
In both methods, the conductor strips, welded on the bus bars, are covered with a thermoplastic insulator resin sheet after the welding.
As mentioned above, the joint section is covered with an insulator resin sheet from both faces thereof. The sheets are then adhered by heated devices, so that not only the conductor strips and the bus bars are firmly bonded, but also the joint section itself is better protected from outside.
The insulator resin sheets may be sized up appropriately, depending on the number of the bus bars and conductor strips used. An increase in the number of circuits can thus be easily met.
Further, adhesion of the insulator resin sheets can be conducted easily by using first and second devices. It is therefore no longer necessary to use a costly resistance welding machine or ultrasonic welding machine. As mentioned above, the devices respectively include a pressing plane with alternating concave and convex configurations. The convex configurations press the insulator resin sheets into the strip gaps formed between the strip layers. The sheets thus inserted between the strip gaps prevent interactions between the conductor strips.
After a first fixing operation is effected by the above devices, a second fixing operation may be performed by piecing, embossing or rivet cramping the insulator resin sheets. Such a second fixing operation strengthens further the adhesion between the conductor elements and the bus bars which had already been adhered firmly by the insulator resin sheets. For example, when the conductor strips and the bus bars are punched from above the resin sheets, there occur returns inside the pierced holes. These returns hold the bus bars and the conductor strips together and solidify the adhesion. Likewise, when the resin sheets are embossed, the conductor strips and the bus bars are undulated, so that they are held together more firmly.
Further, the conductor element and the bus bars may be pierced, and clamped by rivets through the pierced holes. Besides the above mentioned fixing methods, laser welding can also be applied.